The investigation which led to this invention was directed particularly toward stabilizing an emulsion of a liquid (i.e. liquid at room temperature) p-styrenated diphenylamine antioxidant with the following formula: ##STR1## wherein R.sup.4 is selected from phenyl and p-tolyl radicals and R.sup.5 is selected from H and methyl, which changes from its normal creamy white color to a light pastel pink after long storage. The particular styrenated diphenylamine used was Wingstay 29, a trademark of the Goodyear Tire & Rubber Company.
A typical prior art Wingstay 29 emulsion recipe may be found in "Tech Book Facts, Wingstay 29 Anitoxidant in SBR Foam Compounds" a data sheet published by The Goodyear Tire & Rubber Company. Formula 3 of that data sheet is as follows:
______________________________________ Parts by Weight ______________________________________ Wingstay 29 100,00 Oleic Acid 5.00 5% KOH 20.00 H.sub.2 O 75.00 ______________________________________
An emulsion of the above composition can be made as follows:
1. heat the liquid antioxidant to 120.degree.-140.degree. F. to reduce viscosity,
2. add oleic acid,
3. with vigorous agitation, add the KOH (potassium hydroxide) solution slowly, and
4. add the water after inversion has occurred. Inversion is the change from a liquid organic phase in an aqueous phase to an emulsified organic phase in an aqueous phase.
A variety of organic and inorganic chemicals were tested to halt the color formation. The addition of sodium borohydride, sodium hydrosulphite, or citric acid (all reducing agents) destroyed the emulsion. Sodium tetrasulfide, an oxidizing agent caused an immediate yellowing and also destroyed the emulsion. The addition of sodium dimethyldithiocarbonate caused an immediate pinking of the emulsion. Some of the materials which did not affect the change in color were: sodium mercaptobenzothiazole, sodium sulfide, trimethyleneglycol, triisopropanolamine, and sodium nitrite.
After discovery of a class of additives which inhibit and/or reduce color changes in Wingstay 29 emulsions at high pH, experiments were carried out with other protective materials, and the class of compounds amenable to this treatment was broadened to include not only those age resisters similar to Wingstay 29 but also the following age resisters: alkylated diphenyl amine derivatives (semi-staining antioxidants) having the formula: ##STR2## wherein R.sub.1 and R.sub.2 are alkyl radicals having 2 to 12 carbon atoms (2-12C); N-phenyl, N-alkyl-p-phenylenediamines (staining antiozonants) having the formula: ##STR3## wherein R.sub.3 is an alkyl radical (2-8C); and dihydroquinolines (staining antioxidants) having the formula: ##STR4## wherein R.sub.1 is an alkyl radical having 2 to 12 carbon atoms (2-12C):
By the same method of testing, it was learned that the following type of age resisters are not amenable to this treatment:
1. the high temperature reaction product of diphenylamine and acetone, obtained as BLE, a trademark of Uniroyal, Inc.; and
2. N, N'-di-alkyl-p-phenylenediamines (e.g. N,N'-bis (1-ethyl-3-methylpentyl)-p-phenylene diamine).
Further experiments have demonstrated that color improvements can be achieved in the amine age resisters themselves as well as in their emulsions.